Method and apparatus of determining a set of zero correlation zone lengths

ABSTRACT

A method of determining a set of Zero Correlation Zone (ZCZ) lengths, comprises: determining the length of a root sequence, and selecting such a set of ZCZ lengths that, for any cell radius, the maximum number of preambles obtained from a ZCZ length which is selected from the selected set of ZCZ lengths is closest to the maximum number of preambles determined from a ZCZ length which is selected from the set of all integers, wherein the maximum number of preambles is determined from the length of the root sequence and a ZCZ length selected. This disclosure provides a technical solution for selecting a better limited set of ZCZ lengths by which signaling overload is reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/CN2008/070768, filed on Apr. 22, 2008, which claims priority toChinese Patent Application No. 200710074200.1, filed on Apr. 30, 2007;both of which are incorporated by reference herein in their entireties.

TECHNICAL FIELD

The disclosure relates to the technology of mobile communication, andmore particularly, to a method, an apparatus and a mobile communicationsystem of determining a set of Zero Correlation Zone lengths in theRandom Access Preamble (RAP) technology.

BACKGROUND

In a mobile communication system, a Random Access Preamble is normallytransmitted to a base station by a mobile terminal to initiate therandom access procedure and to enable synchronization of the mobileterminal with the base station.

There are 64 preambles in each cell in the document of “3GPP TS 36.211v1.0.0—Physical Channels and Modulation” which was published in March2007. When initiating a random access procedure, a mobile terminaltransmits one of the 64 preambles. A message is transmitted to a basestation by the mobile terminal selecting a particular preamble.

Before transmitting the preamble, a mobile terminal must synchronize tothe carrier frequency and the frame timing of a base station to becomedownlink synchronized. Although the mobile terminal is downlinksynchronized, there is uncertainty when a signal transmitted by themobile terminal arrives at the base station. This is because a mobileterminal far away from the base station will receive downlink signalswith a larger delay than a mobile terminal close to the base station,and the transmitted signals in uplink will take longer time to propagateto the base station for a mobile terminal which is far away from thebase station compared to the signals from a mobile terminal close to thebase station. The uncertainty in round trip time causes interferencesbetween uplink signals transmitted by different mobile terminals unlessuplink synchronization is performed before data transmission in uplink.

The transmission of any of the RAPs allows a base station to estimatethe time of arrival of an uplink signal. The base station can then,based on the time of arrival estimate transmit a time advance command toa mobile terminal to ensure uplink synchronization. Hence, once apreamble is transmitted by a mobile terminal, the base station maydetect which preamble has been transmitted and estimate the time ofarrival.

To obtain good detection properties of the preambles, or to accuratelyestimate the time of arrival of the uplink signal, the set of preamblesshould be designed to have good autocorrelation and cross-correlationproperties.

The set of RAPs in Evolved UTRA (E-UTRA) is defined from one or severalroot sequences. A subset of the preambles x_(u,v)(k) is generated fromthe u^(th) order root Zadoff-Chu (ZC) sequence x_(u)(k) by cyclic shiftsof a plurality of the shift increments N_(CS). Specifically, x_(u,v)(k)may be generated according to the equation below:x _(u,v)(k)=x _(u,v)((k+vN _(CS))mod N _(ZC)),  (1)where v is an integer, and N_(ZC) is the length of the ZC sequencedefined by:x _(u)(k)=W ^(uk(k+1)/2) , k=0, 1, . . . , N _(ZC)−1, W=e ^(−j2πPN)^(ZC) , j=√{square root over (−1)}  (2)

The number of preambles that may be generated from a single rootsequence is N_(pre)=└N_(ZC)/N_(CS)┘, where └n┘ denotes the largestinteger not greater than n. If N_(pre)<64, then several preamble subsetsgenerated from different root sequences are required to obtain 64preambles in a cell. The cross-correlation between different rootsequences is small but still larger than the cross-correlation betweensequences generated by a single root sequence. Thus it is beneficial forthe detection performance to have N_(pre)=64 if N_(pre) could not be setgreater.

The number of ZC sequences contained in each set of ZC sequences withlength of N_(ZC) is N_(ZC)−1. If the number of root sequences forobtaining the 64 preambles of a cell is N_(r), N_(r)=┌64/Npre┐, where┌n┐ denotes the minimal integer not smaller than n, then the number ofdisjoint sets is N_(D)=└(N_(ZC)−1)/N_(r)┘. Different cells in a networkshould make use of preambles obtained from disjoint sets of rootsequences, so that the base station knows whether a transmitted preambleis intended for a certain cell or not. The larger the number of rootsequences N_(r) that is needed for obtaining 64 preambles in a cell, thesmaller is the number of disjoint sets of RAPs N_(D). Thus, from networkplanning perspective it is desirable to have N_(pre)=64, and if that isnot possible to have as high value as possible of N_(pre).

A subset of preambles generated with equation (1) is a set of so-calledZero-Correlation Zone (ZCZ) sequences. The definition for a set of ZCZsequences is as follows: a set of M sequences {d_(v)(k)}, v=0, 1, . . ., M−1, k=0, 1, . . . , N−1, of length N, is said to be a set of ZCZsequences, if all the sequences in the set satisfy the followingautocorrelation and cross-correlation properties:

The periodic autocorrelation function Σ_(k=0)^(N−1)d_(v)(k)d_(v)*((k+p)mod N) is zero for all p such that 0<|p|≦T,and the periodic cross-correlation function Σ_(k=0)^(N−1)d_(v)(k)d_(w)*((k+p)mod N) is zero for all p such that |p|≦T(including p=0), where T is the length of the ZCZ.

A ZC sequence has ideal periodic autocorrelation, for example, Σ_(k=0)^(N−1)x_(u)(k)x_(u)*((k+p)mod N) is zero for all nonzero p. Thus the setof preambles defined as cyclic shifts of the root sequence according toequation (1) is a set of ZCZ sequences, where the ZCZ length isT=N_(CS)−1.

Based on N_(pre)=└N_(ZC)/N_(CS)┘, N_(CS) should be as small as possiblein order to make N_(pre) be as great as possible. But the value ofN_(CS) should not be too small. In a base station a bank of correlatorsare used when receiving RAPs, so that there is one correlator for eachpreamble. Each correlator outputs time of arrival from 0 toT×T_(s)=(N_(CS)−1)×T_(s), where T_(s) is the symbol period of thesequence. The ZCZ property of the set of preambles implies that thecorrelator for any preamble will give a zero output if any otherpreamble is transmitted as long as the sum of the round trip time anddelay spread in the cell is less than or equal to the product of thelength of ZCZ and T_(s) (i.e., T×T_(s)). The maximum round trip timeT_(r) in a cell is given by the cell radius R: T_(r)=2R/c, where c isthe speed of light. Thus, the minimum value of the length of ZCZ and theminimum value of N_(CS) length for a certain cell increase with the cellradius. Therefore, the value of the selected N_(CS) should be largeenough to ensure that the conditions mentioned above are satisfied.

Since the cell radius to be supported in E-UTRA is from 1 km to 100 km,and since N_(CS) should be as small as possible for any given cell,there is a need for multiple values of N_(CS). The value of an N_(CS) ina cell is broadcast to a mobile terminal by a base station. Of course,the base station may broadcast the length of ZCZ to the mobile terminal,so that the mobile terminal knows how to generate preambles. It isdesirable to have as small amount of signaling as possible on thebroadcast channel to save overload. Therefore, to achieve low signalingoverload, there should be a limited predefined set of values of N_(CS)or a set of lengths of ZCZ.

Currently, it is proposed in the 3GPP Tdoc “R1-071661-On constructionand signaling of RACH preambles” disclosed in March 2007 that, thecyclic shift increment value N_(CS) in the cell was proposed to besignalled to the UE but there was no restriction on the values of thecyclic shift increment, which would then give a substantial amount ofsignalling. An alternative proposal is given in the 3GPP Tdoc“R1-071471—Outstanding issues in random access preamble design forE-UTRA” disclosed in March 2007, which is to have 11 values of N_(CS)without specification how to select the values. Of course, it is notdescribed in these documents how to select the lengths of ZCZ either.Currently there is no feasible scheme for selecting an appropriatelimited set of ZCZ lengths, in order to ensure a small and limitedsignaling overload.

SUMMARY

Embodiments of the disclosure provide a method, an apparatus and amobile communication system or determining a set of ZCZ lengths,enabling the select of a limited set of ZCZ lengths good for enhancingthe quality of random access of mobile terminals and for decreasing theoverhead of signaling.

An embodiment of the disclosure provides a method of determining a setof ZCZ lengths, the method comprises: determining a length of a rootsequence; selecting such a set of ZCZ lengths that, for any cell radius,the maximum number of preambles determined from a ZCZ length selectedfrom the selected set of ZCZ lengths, the ZCZ length being applicable tothe cell and being capable of determining a maximum number of preambles,is closest to the maximum number of preambles determined from a ZCZlength selected from the set of all integers, the ZCZ length beingapplicable to the cell and being capable of determining a maximum numberof preambles, wherein the maximum number of preambles is determined fromthe length of the root sequence and a ZCZ length selected.

Another embodiment of the disclosure provides an apparatus ofdetermining a set of ZCZ lengths, the apparatus comprises: a lengthdetermination unit, adapted to determine the length of a root sequence;and a set selection unit, adapted to select such a set of ZCZ lengthsthat, for any cell radius, the maximum number of preambles determinedfrom a ZCZ length selected from the selected set of ZCZ length, the ZCZlength being applicable to the cell and being capable of determining amaximum number of preambles, is closest to the maximum number ofpreambles determined from a ZCZ length selected from the set of allintegers, the ZCZ length being applicable to the cell and being capableof determining a maximum number of preambles, wherein the maximum numberof preambles is determined from the length of the root sequence and aZCZ length selected.

Another embodiment of the disclosure provides a base station, whichincludes: a length determination unit, adapted to determine a length ofa root sequence; and a set selection unit, adapted to select such a setof ZCZ lengths that, for any cell radius, the maximum number ofpreambles determined from a ZCZ length selected from the selected set ofZCZ lengths, the ZCZ length being applicable to the cell and beingcapable of determining a maximum number of preambles, is closest to themaximum number of preambles determined from a ZCZ length selected fromthe set of all integers, being applicable to the cell and being capableof determining a maximum number of preambles, wherein the maximum numberof preambles is determined from the length of the root sequence and aZCZ length selected.

Another embodiment of the disclosure provides a mobile communicationsystem, the system comprises a base station and a mobile terminal, thebase station is adapted to interact with the mobile terminal, and tospecify a ZCZ length from a set of ZCZ lengths for the mobile terminal;the mobile terminal is adapted to generate a preamble according to theZCZ length specified by the base station, and transmitting an uplinksignal to the base station using the preamble; the set of ZCZ lengths issuch a set of ZCZ lengths that, for any cell radius, the maximum numberof preambles determined from a ZCZ length selected from the selected setof ZCZ lengths, the ZCZ length being applicable to the cell and capableof determining a maximum number of preambles, is closest to the maximumnumber of preambles determined from a ZCZ length selected from the setof all integers, the ZCZ length being applicable to the cell and capableof determining a maximum number of preambles, wherein the maximum numberof preambles is determined from the length of the root sequence and aZCZ length selected.

In an embodiment of the disclosure, the limited set of ZCZ lengths beingselected should satisfy a certain condition, providing a technicalsolution for selecting a limited set of ZCZ lengths which is relativegood with decreased signaling overhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating an method embodiment of thedisclosure;

FIG. 2 is a diagram illustrating the relationship between the maximumnumber of preambles and the cell radius according to an embodiment ofthe disclosure;

FIG. 3 is a diagram illustrating the value of maximum relativedifference in the cell radius interval k according to an embodiment ofthe disclosure;

FIG. 4 is a block diagram of the base station according to an embodimentof the disclosure; and

FIG. 5 is a diagram illustrating the mobile communication systemaccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

The general solution of an embodiment of the disclosure is describedfirst, incorporating FIG. 1. As illustrated in FIG. 1, the embodimentincludes:

Step 101: The length of the root sequence is determined;

Step 102: A set of ZCZ lengths is selected so that, for any cell radius,the maximum number of preambles determined from a ZCZ length which isselected from the selected set of ZCZ lengths, and is applicable to thecell and capable of determining a maximum number of preambles, isclosest to the maximum number of preambles obtained from a ZCZ lengthwhich is selected from the set of all integers, and is applicable to thecell and capable of determining a maximum number of preambles, whereinthe maximum number of preambles is determined from the length of theroot sequence and a ZCZ length selected.

In an embodiment of the disclosure, it should be ensured that theproduct of a ZCZ length and the symbol period of the sequence is greaterthan the sum of the round trip time and the delay spread of a cell,i.e., T×T_(s)>T_(r)+T_(d), in which, T is the length of ZCZ, T_(s) isthe symbol period, T_(r) is the round trip time, and T_(d) is the delayspread.

Since the maximum round trip time T_(r) in a cell is determined by thecell radius R, i.e., T_(r)=2R/c, where c is the speed of light,T×T_(s)>T_(r)+T_(d) may be rewritten as T×T_(s)>2R/c+T_(d).

Furthermore, since T=N_(CS)−1, T×T_(s)>2R/c+T_(d) may be rewritten as(N_(CS)−1)×T_(s)>2R/c+T_(d). Therefore, N_(CS)>1+(2R/c+T_(d))/T_(s).

Additionally, since N_(pre)=└N_(ZC)/N_(CS)┘,N_(pre)<└N_(ZC)/(1+(2R/c+T_(d))/T_(s)┘. Thus, N_(pre) may be a functionof the cell radius R. Of course, the cell radius may also be varying;and the value of N_(pre) decreases as the value of N_(CS) increases.

In an embodiment of the disclosure, a limited set of N_(CS) values isconstructed, i.e., for a certain cell radius, the N_(pre) correspondingto the minimum N_(CS) value which is selected from the limited set andis applicable to the cell, is closest to the N_(pre) corresponding tothe minimum N_(CS) value which is selected from the set of all integersand is applicable to the cell. Furthermore, a maximum relativedifference may be constructed from N_(pre). This maximum relativedifference is between the N_(pre)(R), which is determined from theminimum N_(CS) value selected from the set of integers and is applicableto the cell, and the N_(pre)(R), which is determined from the minimumN_(CS) value selected from the limited set and is applicable to thecell. If the finally determined or selected limited set is such a setthat the maximum relative difference between the N_(pre)(R), which isdetermined from the minimum N_(CS) value selected from the set ofintegers and is applicable to the cell, and the N_(pre)(R), which isdetermined from the minimum N_(CS) value selected from the limited setand is applicable to the cell, is minimized in a cell of any radius,this limited set is a required one.

As illustrated in FIG. 2, curve A indicates that for any one cellradius, an integer from the set of all integers may be selected asN_(CS) of the cell, wherein a maximum number of preamble sequences maybe generated based on the integer selected, and the generated preamblesequences are applicable to the cell. Curve B indicates a set of N_(CS)including a limited number of N_(CS). When the limited number of N_(CS)is applied in cells of all radii, within a certain interval of cellradii, a same N_(CS) will be used for all cell radii. Thus, the N_(CS)should be determined according to the maximum cell radius in theinterval of cell radii. Compared with A, the preamble number generatedaccording to B decreases.

Under these conditions, if the selected limited set ensures that themaximum relative difference between the N_(pre)(R) determined from aN_(CS) value selected from any integer and the N_(pre)(R) determinedfrom a N_(CS) value selected from the limited set is minimized, and itis assumed that the N_(pre)(R) determined from a N_(CS) value selectedfrom any integer is A(R) and the N_(pre)(R) determined from a N_(CS)value selected from the limited set is B(R), and then A(R) and B(R) arerespectively illustrated in FIG. 2.

As seen from FIG. 2, there is a small deviation between A(R) and B(R).For a certain cell radius R, the deviation of B(R) from A(R) for somecell radius R may increase the number of required root sequences forthat cell radius R. The increase of the number of root sequences becomesvery important for large cell radii where N_(pre) is small. For example,if A(R)=3 and B(R)=2, the number of root sequences increasessignificantly, from ┌64/3┐=22 to ┌64/2┐=32. An appropriate measure ofthe deviation of B from A should therefore weigh the difference A-B withhigher weight for small N_(pre), e.g. by considering the maximumrelative difference between A(R) and B(R), i.e., [A(R)−B(R)]/A(R). Wewill adopt the maximum relative difference between A(R) and B(R) overall cell radii as the measurement of the deviation of B(R) from A(R),and find a set of N_(CS) values that minimizes this measurement. Thisset may consist of one N_(CS)=0 and K+1 non-zero N_(CS) values. Thetotal number of N_(CS) values in the set is K+2.

For example, in a relatively small cell, it would be possible togenerate 64 ZCZ preambles from a single root sequence if N_(CS)└N_(ZC)=/64┘. This value is the smallest value in the set N_(CS)(k).

The maximum value, N_(CS)(K), is the one that allows for having 2 ZCZsequences from a set single root sequence, so it is └N_(ZC)/2┘.

For the largest cells there is only one RAP generated from each rootsequence. Therefore, N_(CS)(K+1)=0.

The maximum relative difference between A(R) and B(R), i.e.,[A(R)−B(R)]/A(R), is non-increasing with radius R within the interval of[(r(k−1), r(k)] and the interval being k, as illustrated in FIG. 2. InFIG. 2, r(k) denotes the kth cell radius arranged orderly from smallones to large ones. The reason is that B(R) is constant in the interval,whereas A is inversely proportional to the smallest possible N_(CS) forgiven R. This value of N_(CS) increases with the round trip time andhence with R.

If it is assumed that the maximum number of preamble sequences of theset A(R) is N_(pre)(k−1)−1 in the cell radius interval of [(r(k−1),r(k)], the maximum number of preamble sequences of the set B(R)generated in this interval associate with the cell radius r(k), i.e.,the maximum number of preamble sequences is N_(pre)(k). The maximumrelative difference D_(k) in the interval k may be obtained from thefollowing equation:

$D_{k} = \frac{{N_{pre}\left( {k - 1} \right)} - 1 - {N_{pre}(k)}}{{N_{pre}\left( {k - 1} \right)} - 1}$

If D_(k) and N_(pre)(k−1) are given, N_(pre)(k) may be obtained byrearranging the above equation, i.e.:N _(pre)(k)=(1−D _(k))(N _(pre)(k−1)−1)

The maximum relative difference D_(max) for all cell radii may be givenby D_(max)=max {D_(k)}_(k=1) ^(K).

For N_(pre)(k), we will first allow N_(pre)(k) to be a real number, andthen round the result to the nearest integer. Additionally, N_(pre)(0)and N_(pre)(K) are fixed.

Then D_(max) is minimized if all D_(k) are equal, i.e. D_(k)=D, k=1, 2,. . . , K, as will be proved in the following.

A set of values, {N_(pre) ⁽¹⁾(k)}_(k=0) ^(K), is constructed with theconstraint that N_(pre) ⁽¹⁾(k)=N_(pre)(k) for k=0 and k=K, so that D_(k)⁽¹⁾=D, k=1, 2, . . . , K. For this set, D_(max)=D.

Next, another set of values, {N_(pre) ⁽²⁾(k)}_(k=0) ^(K), is constructedwith the constraint that N_(pre) ⁽²⁾(k)=N_(pre)(k) for k=0 and k=K, sothat D_(max)<D, i.e. D_(k) ⁽²⁾<D_(k) ⁽¹⁾, k=1, 2, . . . , K.

When k=1, since D_(k) ⁽²⁾<D_(k) ⁽¹⁾ and N_(pre) ⁽²⁾(0)=N_(pre) ⁽¹⁾(0),N_(pre) ⁽²⁾(1)>N_(pre) ⁽¹⁾(1) is obtained according toN_(pre)(k)=(1−D_(k))(N_(pre)(k−1)−1).

When k=2, since D₂ ⁽²⁾<D₂ ⁽¹⁾ and N_(pre) ⁽²⁾(1)>N_(pre) ⁽¹⁾(1), N_(pre)⁽²⁾(2)>N_(pre) ⁽¹⁾(2) is obtained according toN_(pre)(k)=(1−D_(k))(N_(pre)(k−1)−1).

Similarly, for all k, since N_(pre) ⁽²⁾(K)=N_(pre) ⁽¹⁾(K)=N_(pre)(K),N_(pre) ⁽²⁾(k)>N_(pre) ⁽¹⁾(k) is impossible.

Thus, it is impossible to construct a set of values N_(pre)(k) such thatD_(max)<D, which proves that D_(max) is minimized if all D_(k) areequal, i.e. D_(k)=D, k=1, 2, . . . , K.

In this way, the set of values {N_(pre)(k)}_(k=0) ^(K) which minimizesD_(max) may be found.

Replacing D_(k) by D in N_(pre)(k)=(1−D_(k))(N_(pre)(k−1)−1) andrearranging the equation, a linear difference equation is obtained asfollows:N _(pre)(k)−aN _(pre)(k−1)=−a, wherein a=(1−D).

By recursion, it is obtained from the above equation:

$\begin{matrix}{{N_{pre}(k)} = {{{N_{pre}(0)}a^{k}} + {\frac{a}{1 - a}\left( {a^{k} - 1} \right)}}} & (3)\end{matrix}$

From the above equation and the boundary conditions N_(pre)(0) andN_(pre)(K), a may be determined numerically.

For example, the maximum number of preambles generated from one rootsequence is 64, i.e., N_(pre)(0)=64. The minimum number of preambleobtained by cyclic shift is 2, for example, N_(pre)(14)=2. Thus, a=0.856may be obtained from these two parameters, and all N_(pre)(k), k=1, 2, .. . may further be obtained.

The maximum relative difference is minimized through an approximateminimization by a sub-optimal algorithm, i.e., by minimizing the maximumrelative difference for fictive real-valued maximum number of ZCZ RAPs,and the maximum number of the ZCZ RAPs is thereafter quantized. Themethod is specified below.

By first rounding the fictive real-valued N_(pre)(k) in

${{N_{pre}(k)} = {{{N_{pre}(0)}a^{k}} + {\frac{a}{1 - a}\left( {a^{k} - 1} \right)}}},$the following equation is obtained:N _(CS)(k)=└N _(ZC) /[N _(pre)(0)×a ^(k) +a/(1−a)×(a ^(k−)1)]┘  (4)where └X┘ denotes the maximum integer not greater than x, N_(ZC) is thelength of the root sequence, N_(pre)(0) denotes the maximum number ofpreambles generated from the root sequence.

Still taking the above example as an example, if N_(pre)(0)=64 andN_(pre)(14)=2, a=0.856 is obtained based on equation (3). Next, whenN_(ZC)=839, N_(cs)(k), k=0, 1, 2, . . . , 14 obtained based on equation(4) is illustrated in table 1:

TABLE 1 k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 7 46 8 59 9 76 1093 11 119 12 167 13 279 14 419

If only one preamble sequence is obtained for a very large cell, whichis the sequence itself, then N_(CS)=0. Adding this value into the abovetable, table 2 is obtained:

TABLE 2 k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 7 46 8 59 9 76 1093 11 119 12 167 13 279 14 419 15 0

Finally, the true integer value of N_(pre)(k) is obtained fromN_(pre)(k)=└N_(ZC)/N_(CS)(k)┘ that for some values of k N_(ZC)/N_(CS)(k)are greater than the rounded values N_(pre)(k). As illustrated in FIG.3, when K=14, the value of D_(k) obtained from the real number value ofN_(pre)(k) is D=0.144. It can be seen from FIG. 3 that the true integervalues of N_(pre)(k) will cause D_(k) to deviate from D. But thedeviation is still very small for all cells except the two largestcells. Thus, the selected limited set of values of N_(CS) is applicable.

It should be noted that if the limited set of values of N_(CS) isdetermined, the limited set of lengths of ZCZ may also be determined,for instance, according to T=N_(CS)−1.

Correspondingly, the disclosure provides an embodiment of an apparatusof determining a set of ZCZ lengths. As illustrated in FIG. 4, theapparatus includes: a length determination unit 410, configured todetermine a length of a root sequence; and a set selection unit 420,configured to select such a set of ZCZ lengths that, for any cellradius, the maximum number of preambles determined from a ZCZ lengthwhich is selected from the selected set of ZCZ lengths, and isapplicable to the cell and capable of determining a maximum number ofpreambles, is closest to the maximum number of preambles determined froma ZCZ length which is selected from the set of all integers, and isapplicable to the cell and capable of determining a maximum number ofpreambles, wherein the maximum number of preambles is determined by thelength of the root sequence and a ZCZ length selected.

The set selection unit 420 may include: a module 421 adapted for theselection of a set of cyclic shift increments, wherein, the module 421is configured to select such a set of cyclic shift increments that, forany cell radius, the maximum number of preambles determined from acyclic shift increment which is selected from the selected set of cyclicshift increments, and is applicable to the cell, is closest to themaximum number of preambles determined from a cyclic shift incrementwhich is selected from the set of all integers and is applicable to thecell, wherein the maximum number of preambles is determined by the rootsequence length and a cyclic shift increment selected; and a module 422adapted to obtain a set of ZCZ lengths, wherein the module is configuredto obtain the set of ZCZ lengths according to the selected set of cyclicshift increments.

In above apparatus embodiment, the cyclic shift increment selected fromthe selected set of cyclic shift increments is the minimum cyclic shiftincrement in the selected set of cyclic shift increments; and the cyclicshift increment selected from the set of all integers is the minimumcyclic shift increment in the set of all integers.

The disclosure provides an embodiment of a base station, as illustratedin FIG. 4, which includes: a length determination unit 410, configuredto determine a length of a root sequence; and a set selection unit 420,configured to select such a set of ZCZ lengths that, for any cellradius, the maximum number of preambles determined from a ZCZ lengthwhich is selected from the selected set of ZCZ lengths, and isapplicable to the cell and capable of determining a maximum number ofpreambles, is closest to the maximum number of preambles determined froma ZCZ length which is selected from the set of all integers, and isapplicable to the cell and capable of determining a maximum number ofpreambles, wherein the maximum number of preambles is determined fromthe length of the root sequence and a ZCZ length selected.

The disclosure further provides an embodiment of a mobile communicationsystem, as illustrated in FIG. 5. The system comprises a base station400 and a mobile terminal 500. The base station 400 is configured tointeract with the mobile terminal 500, and to specify a ZCZ length froma set of ZCZ lengths for the mobile terminal 500; the mobile terminal500 is configured to generate a preamble according to the ZCZ lengthspecified by the base station 400, and to transmit an uplink signal tothe base station 400 using the preamble; the set of ZCZ lengths is sucha set of ZCZ lengths that, for any cell radius, the maximum number ofpreambles determined from a ZCZ length which is selected from theselected set of ZCZ lengths, and is applicable to the cell and capableof determining a maximum number of preambles, is closest to the maximumnumber of preambles determined from a ZCZ length which is selected fromthe set of all integers, and is applicable to the cell and capable ofdetermining a maximum number of preambles, wherein the maximum number ofpreambles is determined from the length of the root sequence and a ZCZlength selected.

In above embodiment of the mobile communication system, the cyclic shiftincrement selected from the selected set of cyclic shift increments isthe minimum cyclic shift increment applicable to the cell in theselected set of cyclic shift increments, the cyclic shift incrementselected from the set of all integers is the minimum cyclic shiftincrement applicable to the cell in the set of all integers.

In general, in embodiments of the disclosure, the selected limited setof N_(CS) values should be such a set that, in a plurality of intervalsof cell radii, the maximum relative difference between the maximumnumber of the ZCZ RAPs determined from the minimum N_(CS) value of thelimited set, which is applicable to the plurality of cells, and themaximum number of the ZCZ RAPs determined from a plurality of N_(CS)values of a set of integers which are applicable to the plurality ofcells is minimized. Furthermore, a limited set of ZCZ lengths may beselected. Of course, in a plurality of intervals of cell radii, themaximum relative difference between the maximum number of the ZCZ RAPsdetermined from the minimum ZCZ length of the limited set of ZCZlengths, which is applicable to the plurality of cells, and the maximumnumber of the ZCZ RAPS determined from a plurality of ZCZ lengths of theset of all integers which are applicable to the plurality of cells isminimized.

What are described above are only preferred embodiments of thedisclosure. It should be noted that, for a person skilled in the art,variations and improvements may be made without deviating from theprinciple of the disclosure. Those variations and improvements are allregarded to be within the scope of the disclosure.

The invention claimed is:
 1. A method of determining a set of ZeroCorrelation Zone, ZCZ, lengths of a mobile communication system,comprising: selecting, by an appatatus of the mobile communicationsystem, a set of cyclic shift increments N_(CS)(k), the set of cycleshift increments N_(CS)(k) comprising one cyclic shift incrementsN_(CS)=0; determining by the apparatus, a set of ZCZ lengths bydecreasing each non-zero cyclic shift increment of the set of cyclicshift increments N_(CS)(k) by 1, non-zero cyclic shift increments of theset of cyclic shift increments N_(CS)(k) being generated in accordancewith the following formula:N _(CS)(k)=└N _(ZC) /[N _(pre)(0)×a ^(k) +a/(1−a)×(a ^(k)−1)]┘, k=0, 1,2 . . . 14; where a=0.856, N_(pre)(0)=64 and N_(ZC)=839.
 2. The methodaccording to claim 1, wherein the set of cyclic shift increments beingapplicable to the plurality of intervals corresponding to cell radii ofthe communication system, a first cyclic shift increment from the set ofcyclic shift increments corresponds to a second cyclic shift incrementfrom a set of all integers, the first cyclic shift increment beingapplicable to an interval of the plurality of intervals and beingcapable of determining the first maximum number of preambles for theinterval, the second cyclic shift increment being applicable in theinterval and being capable of determining a second maximum number ofpreambles in the interval, the number of elements of the set of cyclicshift increments is less than the number of elements of the set of allintegers, the first maximum number of preambles is determined inaccordance with the length of a root sequence and the first cyclic shiftincrement, the first maximum number of preambles is determined inaccordance with the length of a root sequence and the second cyclicshift increment, and maximum relative difference between the firstmaximum number of preambles determined from a first cyclic shiftincrement and the second maximum number of preambles determined from asecond cyclic shift increment is minimized over the plurality ofintervals.
 3. The method according to claim 2, wherein the maximumrelative difference is minimized by performing an approximateminimization by a sub-optimal algorithm.
 4. The method according toclaim 2, wherein: the maximum relative difference is minimized forfictive real-valued maximum number of ZCZ Random Access Preambles, ZCZRAPs, and the fictive real-valued maximum number of the ZCZ RAPs isquantized.
 5. The method according to claim 2, wherein the maximumrelative difference is minimized in the following way: the maximumrelative difference between the first maximum number of preamblesdetermined from a first cyclic shift increment and the second maximumnumber of preambles determined from a second cyclic shift increment isthe same for all the intervals.
 6. The method according to claim 2,wherein the first maximum number of preambles is generated with thefollowing formula:${{N_{pre}(k)} = {{{N_{pre}(0)}a^{k}} + {\frac{a}{1 - a}\left( {a^{k} - 1} \right)}}},{k = 1},{2\mspace{14mu}\ldots}\mspace{14mu},{14:}$wherein N_(pre)(0) is the maximum number of preambles generated from theroot sequence, a=(1−D), and D is the maximum relative difference betweenthe first maximum number of preambles and the second maximum number ofpreambles.
 7. The method according to claim 6, wherein a=0.856 isobtained when N_(pre)(0)=64 and N_(pre)(14)=2.
 8. The method accordingto claim 2, wherein the maximum number of preambles is obtained for asingle root sequence.
 9. The method according to claims 2, wherein theroot sequence is a Zadoff-Chu sequence.
 10. The method of according toclaim 2 wherein: the first cyclic shift increment selected from the setof cyclic shift increments is an applicable minimum cyclic shiftincrement in the set of cyclic shift increments, and the second cyclicshift increment selected from the set of all integers is an applicableminimum cyclic shift increment in the set of all integers.
 11. Themethod according to claim 2, wherein the product of a ZCZ length in theset of ZCZ lengths and a symbol period is greater than the sum of around trip time and a delay spread of a cell.
 12. A method ofdetermining a set of Zero Correlation Zone, ZCZ, lengths of a mobilecommunication system, comprising: selecting, by an apparatus of themobile communication sysem, a pre-defined set of cyclic shift incrementsN_(CS)(k); determining, by the apparatus, a set of ZCZ lengths bydecreasing each non-zero cyclic shift increment of the pre-defined setof cyclic shift increments N_(CS)(k) by 1, the pre-defined set of cyclicshift increments N_(CS)(k) including all of the non-zero cyclic shiftincrements generated by the specific listing of k values provided in thefollowing table: k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 7 46 859 9 76 10 93 11 119 12 167 13 279 14
 419.


13. The method according to claim 12, wherein non-zero cyclic shiftincrements are generated with the following formula:N _(CS)(k)=└N _(ZC) /[N _(pre)(0)×a ^(k) +a/(1−a)×(a ^(k)−1)]┘, k=0, 1 ,2 . . . , 14; where a=0.856, N_(pre)(0)=64 and, N_(ZC)=839.
 14. Anapparatus for determining a set of Zero Correlation Zone, ZCZ, lengthsof a mobile communication system, comprising: a unit, adapted to selecta set of cyclic shift increments N_(CS)(k); and a unit, adapted todetermine a set of ZCZ lengths by decreasing each non-zero cyclic shiftincrement of pre-defined set of cyclic shift increments by N_(CS)(K) 1,the predefined set of cyclic shift increments N_(CS)(K) including all ofthe non-zero cyclic shift increments generated by the specific listingof k values provided in the following table: k N_(CS)(k) 0 13 1 15 2 183 22 4 26 5 32 6 38 7 46 8 59 9 76 10 93 11 119 12 167 13 279 14
 419.


15. A mobile communication system, comprising a base station, whereinthe base station comprises: an apparatus configured to determine a setof Zero Correlation Zone, ZCZ, lengths by decreasing each nonzero cyclicshift increment of a pre-defined set of cyclic shift incrementsN_(CS)(k) by 1, the pre-defined set of cyclic shift increments N_(CS)(k)being used for generating random access preambles, and the pre-definedset of cyclic shift increments N_(CS)(k) including all of the non-zerocyclic shift increments generated by the listing of k values provided inthe following table: k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 7 468 59 9 76 10 93 11 119 12 167 13 279 14 419

and a unit, adapted to specify a ZCZ length from the set of ZCZ lengthsfor a mobile terminal.
 16. A communication system, comprising a basestation in communication with a mobile terminal, wherein the basestation is arranged to receive from the mobile terminal a signalcomprising a random access preamble with a Zero Correlation Zone lengthfrom a set of ZCZ lengths, the set of ZCZ lengths being determined bydecreasing each non-zero cyclic shift increment of a pre-defined set ofcyclic shift increments N_(CS)(k) by 1, and the pre-defined set ofcyclic shift increments N_(CS)(k) including all of the non-zero cyclicshift increments generated by the specific listing of k values providedin the following table: k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 746 8 59 9 76 10 93 11 119 12 167 13 279 14
 419.


17. A communication system, comprising a mobile terminal incommunication with a base station, wherein the mobile terminal isarranged to transmit to the base station a signal comprising a randomaccess preamble with a Zero Correlation Zone length from a set of ZCZlengths, the set of ZCZ lengths being determined by decreasing eachnon-zero cyclic shift increment of a pre-defined set of cyclic shiftincrements N_(CS)(k) by 1, and the pre-defined set of cyclic shiftincrements N_(CS)(k) including all of the non-zero cyclic shiftincrements generated by the specific listing of k values provided in thefollowing table: k N_(CS)(k) 0 13 1 15 2 18 3 22 4 26 5 32 6 38 7 46 859 9 76 10 93 11 119 12 167 13 279 14 419.